Thus J.J.Thomson shows that if, in certain conditions, a conductor is placed in a magnetic field, the ions have to describe an epicycloid, and their journey is thus lengthened, while the electric resistance must increase.

If the field is in the direction of the displacement, they describe helices round the lines of force and the resistance is again augmented, but in different proportions.

Various experimenters have noted phenomena of this kind in different substances.
For a long time it has been noticed that a relation exists between the calorific and the electric conductivity; the relation of these two conductivities is sensibly the same for all metals.

The modern theory tends to show simply that it must indeed be so.

Calorific conductivity is due, in fact, to an exchange of electrons between the hot and the cold regions, the heated electrons having the greater velocity, and consequently the more considerable energy.